Recently, along with the trend of higher frequency of mobile communication, the operating frequency of a surface acoustic wave device (hereinafter called SAW device) is becoming higher, ranging from hundreds of MHz to several GHz. Moreover, in the antenna duplexer in which a large dielectric filter was used hitherto in order to have a sufficient electric power tolerance, it is being studied to apply the SAW device in the filter section to meet the needs of reduction of size of terminal device. However, due to increase in the operating frequency and operating power, the stress applied to the comb type electrode (IDT) of the SAW device increases, and the SAW device using the conventional electrode material had a problem in the electric power tolerance.
To solve this problem, various electrode materials have been reported, and are known to have a better electric power tolerance than the existing electrode material of Cl--Cu alloy having Cu added to Al. One of them is to use Ti, Ge, Pd or the like, instead of Cu, as the additive to Al, and to improve the electric power tolerance (for example, Japanese Laid-open Patent No. 63-278343 and Japanese Laid-open Patent No. 2-274008).
Considering, for instance, the AMPS standard which requires guarantee of operation for more than 50,000 hours by applying power of 1 W or more in the filter of the antenna duplexer, all these materials hardly satisfy the electric power tolerance sufficiently.
On the other hand, as disclosed in International Laid-open Patent No. WO97/11526, segregation of the additive Cu into the Al grain boundary by aging of Cl--Cu alloy, as known well in semiconductors, is also effective in the SAW device. In this case, too, improvement in electric power tolerance after aging is at most about several times, and the electric power tolerance is not improved sufficiently for use in the antenna duplexer.
As other means, it is attempted to improve the electric power tolerance significantly by using an electrode of three-layer structure having Cu inserted in the middle of Cl--Cu alloy, as unveiled in Japanese Laid-open Patent No. 7-122961, and in this method, although the electric power tolerance can be significantly improved, since there is the Cu layer having a specific gravity of 3.3 times that of Al in a portion of the electrode, a very strict precision of film thickness is required in manufacture, as compared with the ordinary Al (or Cl--Cu alloy), in the SAW device in which the weight of the electrode has a very large effect on the electric characteristics. Besides, since the Cu layer is used, when forming the electrode pattern, it is hard to apply the ordinary dry etching, and difficult problems exist in the manufacture and in the manufacturing yield.
Japanese Laid-open Patent No. 9-223944 is proposed to solve these problems, that is, the electric power tolerance is improved notably by the electrode formed by laminating a first film composed of an Al film or a film adding at least one other element to Al, and a second film composed of a metal of which coefficient of diffusion to Al is larger than the coefficient of self diffusion of Al. In particular, when Mg is used as the second film, it is known to solve the manufacturing problems experienced in the above case of the electrode of three-layer structure using Cl--Cu and Cu.
In this constitution, however, since the second film composed of a metal of which coefficient of diffusion to Al is larger than the coefficient of self diffusion of Al, and Al are laminated, in the heating process after forming the film, diffusion and alloying reaction may progress between Al and the metal used in the second film, the specific resistance of the electrode may be increased. Elevation of specific resistance of the electrode has a large effect on the insertion loss of the SAW device, and it is a serious problem, in particular, when fabricating a transmission side filter of the antenna duplexer in which low loss is demanded.
In addition, along with the trend of higher frequency, the line width of the IDT electrode becomes narrower, and the SAW device in which a large electric power is applied is raised in the chip temperature during operation, and a problem is also involved in the stability of electric characteristics of the SAW device. In particular, as compared with Cu, Mg is high in solid solution limit to Al, and the increase rate of specific resistance is high when incorporated in Al, and this tendency is more manifest than in the case of using Cu.
It is hence an object of the invention to present a surface acoustic wave device using an electrode excellent in electric power tolerance, compatible with the conventional pattern forming technique, stable thermally, and capable of preventing increase of insertion loss, its manufacturing method, and a mobile communication equipment using the same.